title: The association of birth order with later body mass index and blood pressure: a comparison between prospective cohort studies from the United Kingdom and Brazil.
abstract: BACKGROUND
  Previous studies have found greater adiposity and cardiovascular risk in first born children. The causality of this association is not clear. Examining the association in diverse populations may lead to improved insight.
  
  
  METHODS
  We examine the association between birth order and body mass index (BMI), systolic and diastolic blood pressure (SBP/DBP) in the 2004 Pelotas cohort from southern Brazil and the Avon Longitudinal Study of Parents and Children (ALSPAC) from Bristol, south-west England, restricting analysis to families with two children in order to remove confounding by family size.
  
  
  RESULTS
  No consistent differences in BMI, SBP or DBP were observed comparing first and second born children. Within the Pelotas 2004 cohort, first born females were thinner, with lower SBP and DBP; for example, mean difference in SBP comparing first with second born was -0.979 (95% confidence interval -2.901 to 0.943). In ALSPAC, first born females had higher BMI, SBP and DBP. In both cohorts, associations tended to be in the opposite direction in males, although no statistical evidence for gender interactions was found.
  
  
  CONCLUSIONS
  The findings do not support an association between birth order and BMI or blood pressure. Differences to previous studies may be explained by differences in populations and/or confounding by family size in previous studies.
authors.affiliation: 1] MRC Integrative Epidemiology Unit at the University of Bristol, Bristol, UK [2] School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors.forename: L D
authors.initials: LD
authors.lastname: Howe
authors.affiliation: Postgraduate Programme in Epidemiology, Federal University of Pelotas, Pelotas, Brazil.
authors.forename: P C
authors.initials: PC
authors.lastname: Hallal
authors.affiliation: Postgraduate Programme in Epidemiology, Federal University of Pelotas, Pelotas, Brazil.
authors.forename: A
authors.initials: A
authors.lastname: Matijasevich
authors.affiliation: Institute of Child Health, University College London, London, UK.
authors.forename: J C
authors.initials: JC
authors.lastname: Wells
authors.affiliation: Postgraduate Programme in Epidemiology, Federal University of Pelotas, Pelotas, Brazil.
authors.forename: I S
authors.initials: IS
authors.lastname: Santos
authors.affiliation: Postgraduate Programme in Epidemiology, Federal University of Pelotas, Pelotas, Brazil.
authors.forename: A J D
authors.initials: AJ
authors.lastname: Barros
authors.affiliation: 1] MRC Integrative Epidemiology Unit at the University of Bristol, Bristol, UK [2] School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors.forename: D A
authors.initials: DA
authors.lastname: Lawlor
authors.affiliation: Postgraduate Programme in Epidemiology, Federal University of Pelotas, Pelotas, Brazil.
authors.forename: C G
authors.initials: CG
authors.lastname: Victora
authors.affiliation: 1] MRC Integrative Epidemiology Unit at the University of Bristol, Bristol, UK [2] School of Social and Community Medicine, University of Bristol, Bristol, UK.
authors.forename: G D
authors.initials: GD
authors.lastname: Smith
mesh_terms: D050154:Adiposity; D000293:Adolescent; D000328:Adult; D000367:Age Factors; D001722:Birth Order; D001724:Birth Weight; D001794:Blood Pressure; D001827:Body Height; D015992:Body Mass Index; D001938:Brazil / Q000453:epidemiology; D002318:Cardiovascular Diseases / Q000208:ethnology / Q000209:etiology / Q000517:prevention & control*; D002648:Child; D005191:Family Characteristics; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D063766:Pediatric Obesity / Q000208:ethnology / Q000209:etiology / Q000517:prevention & control*; D011446:Prospective Studies; D012307:Risk Factors; D012959:Socioeconomic Factors; D006113:United Kingdom / Q000453:epidemiology